This invention relates to a differential amplifier and, more specifically, to a differential amplifier with an input terminal supplied with an input signal having an input voltage which has an input D.C. level of a reference voltage such as a zero voltage.
Such a differential amplifier is used to supply a double balanced mixer (DBM) with a pair of signals which have normal and inverted phases, respectively. The differential amplifier is also used as a first stage amplifier of a D.C. directly connected amplifier device comprising a plurality of differential amplifiers as unit amplifiers in order to be able to afford for dispersion of characteristics of elements which compose the D.C. directly connected amplifier device.
Various differential amplifiers are already known. For example, a differential amplifier is reported by Masafumi SHIGAKI et al in a Japanese technical paper contributed to "Densi Tusin Gakkai Gijutsu Kenkyu Hokoku (Report of Technical Study of the Institute of Electronics and Communication Engineers of Japan)" as Paper No. SSD85-115 under the title of "GaAs Monolithic Front End IC" according to contributors' translation. The differential amplifier disclosed by SHIGAKI et al., is implemented by an integrated circuit.
In the manner which will later be described, a conventional differential amplifier of the type described is for use in combination with a pair of level shift circuits. The conventional differential amplifier has first and second power supply terminals supplied with positive and negative source potentials, respectively. An earth terminal is supplied with a zero voltage. A reference terminal is supplied with the zero voltage. First and second output terminals are for producing first and second output signals having first and second output voltages, respectively. Each of the first and the second output voltages is representative of a difference between the input voltage and the zero voltage. The first and the second output voltages have inverted polarities relative to each other. In the conventional differential amplifier, the reference terminal is directly connected to the earth terminal.
The conventional differential amplifier is defective in that its gain decreases in a high frequency region for the reason which will later be described in conjunction with one of a few figures of the accompanying drawing.